A brake system is one of the primary safety features in every road vehicle. The ability to quickly decelerate and bring a vehicle to a controlled stop is critical to the safety of the vehicle occupants and those in the immediate vicinity. For this reason, a vehicle braking system is designed and manufactured to exacting specifications and is subject to rigorous inspection.
Disc brake assemblies, which are typically mounted on the front wheels of most passenger vehicles, are primary components of a brake system. Generally, a disc brake assembly includes a caliper that cooperates with a brake hydraulic system, a pair of brake pads, a hub, and a rotor. The caliper supports and positions the pair of brake pads on opposing sides of the brake rotor. In a hubless brake rotor (i.e. when the rotor and hub are separate components), the rotor is secured to the vehicle hub with a series of bolts and a rotor hat. The rotor rotates with the hub about a vehicle spindle axis. When a vehicle driver depresses a brake pedal to activate the hydraulic system, the brake pads are forced together and toward the rotor to grip friction surfaces of the rotor.
Disc brake assemblies must be maintained to the manufacturer's specifications throughout the life of the vehicle to assure optimum performance and maximum safety. However, several problems have plagued the automotive industry since the inception of disc brakes.
A significant problem in brake systems is usually referred to as "lateral runout." Generally, lateral runout is the side-to-side movement of the friction surfaces of the rotor as the rotor rotates with the vehicle hub about a spindle axis. Referring to FIG. 1, for example, a rotor having friction surfaces on its lateral sides is mounted on a vehicle hub for rotation about the horizontal spindle axis X. In an optimum rotor configuration, the rotor is mounted to rotate in a plane Y that is precisely perpendicular to the spindle axis X. Generally, good braking performance is dependant upon the rotor friction surfaces being perpendicular to the spindle's axis of rotation X and being parallel to one another. In the optimum configuration, the opposing brake pads contact the friction surfaces of the rotor at perfect 90 degree angles and exert equal pressure on the rotor as it rotates. More typically, however, the disc brake assembly produces at least a degree of lateral runout that deviates from the ideal configuration. For example, a rotor often will rotate in a canted plane Y' and about an axis X' that is a few thousandths of an inch out of axial alignment with the spindle (shown in exaggerated fashion in FIG. 1). In this rotor configuration, the brake pads, which are perpendicular to the spindle axis X, do not contact the friction surfaces of the rotor along a constant pressure plane.
The lateral runout of a rotor is the lateral distance that the rotor deviates from the ideal plane of rotation Y during a rotation cycle. A certain amount of lateral runout is inherently present in the hub and rotor assembly. This lateral runout often results from defects in individual components. For example, rotor friction surface runout results when the rotor friction surfaces are not perpendicular to the rotor's own axis of rotation, rotor hat runout results when the hat connection includes deviations that produce an off center mount, and stacked runout results when the runouts of the components are added or "stacked" with each other. An excessive amount of lateral runout in a component or in the assembly (i.e., stacked runout) will generally result in brake noise, pedal pulsation, and a significant reduction in overall brake system efficiency. Moreover, brake pad wear is uneven and accelerated with the presence of lateral runout. Typically, manufacturers specify a maximum lateral runout for the friction surfaces, rotor hat, and hub that is acceptable for safe and reliable operation.
After extended use, a brake rotor must be resurfaced to bring the brake assembly within manufacturers' specifications. This resurfacing is typically accomplished through a grinding or cutting operation. Several prior art brake lathes have been used to resurface brake rotors. These prior art lathes can be categorized into three general classes: (1) bench-mounted lathes; (2) on-car caliper-mounted lathes; and (3) on-car hub-mounted lathes.
In general, bench-mounted lathes are inefficient and do not have rotor matching capabilities. To resurface a rotor on a bench-mounted lathe, the operator is first required to completely remove the rotor from the hub assembly. The operator then mounts the rotor on the bench lathe using a series of cones or adaptors. After the cutting operation, the operator remounts the rotor on the vehicle spindle. Even if the rotor is mounted on the lathe in a perfectly centered and runout-free manner, the bench lathe resurfacing operation does not account for runout between the rotor and hub. In addition, bench lathes are susceptible to bent shafts which introduce runout into a machined rotor. This runout is then carried back to the brake assembly where it may combine with hub runout to produced a stacked runout effect.
Similarly, caliper-mounted lathes have had limited success in compensating for lateral runout, and require time consuming manual operations. During a rotor resurfacing procedure, the brake caliper must be removed to expose the rotor and hub. Once this is done, the caliper mounting bracket is used to mount the on-car caliper-mounted lathe. Caliper-mounted lathes lack a "rigid loop" connection between the driving motor and cutting tools, and are unable to assure a perpendicular relationship between the cutting tools and the rotor. Nor does a typical caliper-mounted lathe have a reliable means for measuring and correcting lateral runout. Typically, such lathes use a dial indicator to determine the total amount of lateral runout in the disc assembly. This measurement technique is problematic in terms of time, accuracy, and ease of use.
On-car hub-mounted lathes, generally are the most accurate and efficient means for resurfacing the rotor. Such a lathe is disclosed in U.S. Pat. No. 4,226,146, which is incorporated by reference.
Referring now to FIG. 2, an on-car disc brake lathe 10 may be mounted to the hub of a vehicle 14. The lathe 10 includes a body 16, a driving motor 18, an adaptor 20, and a cutting assembly 22 including cutting tools 23. The lathe may be used with a stand or an anti-rotation post (not shown), either of which can counter the rotation of the lathe during a resurfacing operation. After the brake caliper is removed, the adaptor 20 is secured to the hub of the vehicle 14 using the wheel lug nuts. The lathe body 16 is then mounted to the adaptor 20, the orientation of which may be adjusted using adjustment screws 24.
The operator then determines the total amount of lateral runout and makes an appropriate adjustment. Specifically, the operator mounts a dial indicator 26 to the cutting head 22 using a knob 28. The dial indicator 26 is positioned to contact the vehicle 14 at one of its distal ends as shown in FIG. 2. Once the dial indicator 26 is properly positioned, the operator takes the following steps to measure and compensate for lateral runout:
(1) The operator mates the lathe to the rotor using the adaptor.
(2) The operator activates the lathe motor 18, which rotates the adaptor 20, the brake assembly hub, and the rotor. The total lateral runout of the assembly is reflected by corresponding lateral movement in the lathe body.
(3) The lateral movement of the lathe body is then quantified using the dial indicator 26. Specifically, the operator observes the dial indicator to determine the high and low deflection points and the corresponding location of these points on the lathe.
(4) Upon identifying the highest deflection of the dial indicator, the operator stops the rotation at the point of the identified highest deflection.
(5) The operator then adjusts the lathe to compensate for runout of the assembly. This is accomplished by careful turning of the adjustment screws 24. There are four adjustment screws. The screw or screws to be turned depend on the location of the high deflection point. Turning the screws adjusts the orientation of the lathe body with respect to the adaptor 20 (and therefore with respect to the rotor and hub) to mechanically compensate for the runout of the assembly. The operator adjusts the screws until the highest deflection point is reduced by half as determined by reference to the dial indicator 26.
(6) The operator activates the lathe motor 18 and observes the dial indicator 26 to again identify the highest deflection of the dial. If the maximum lateral movement of the lathe body, as measured by the needle deflection, is acceptable (i.e. typically less than 3/1000 of an inch) then mechanical compensation is complete and the lathe resurfacing operation can commence. Otherwise, further measurement and adjustment is made by repeating steps (1) to (6). The resurfacing operation is then performed by adjusting the tool holder 22 and cutting tools 23 to set the proper cutting depth.
Although the hub mounted on-car brake lathe was a considerable advance over prior brake lathes, its structure and the corresponding procedure for compensating for lateral runout of the disc brake assembly has practical limitations. First, the procedure requires a significant amount of time to determine and adjust for lateral runout of the brake assembly. Although the specific amount of time necessary will vary based upon operator experience, the time for even the most experienced operator is significant and can substantially increase the cost associated with rotor resurfacing. Second, the procedure requires extensive education and operator training to assure that proper mechanical compensation for lateral runout is accomplished. Moreover, the accuracy and success of measurement and adjustment of lateral runout will vary from operator to operator.